Growing up in the dawn of Computer Games, the distance between being a modder and a player wasn't much. Typing in a game from a book titled "101 Basic Computer Games" it is a rather natural step to change some variables and then to add whole new functions. A few thousand lines of BASIC later and you weren't a modder, you were a studio.
As computer games advanced I didn't bother with reading the rule books. Figuring out the rules is half the fun and understanding how the games worked was the other half. Growing up a computer game player as computer games grew up, I maintained the ability to change the rules.
Modding is part of playing the game. Card games at my grandma's house involved knowing the rules grandma added. Mom would change rules to make puzzles and games harder, and dad who played strategy games changed rules to make them more realistic. Almost any gamer could change the rules of any game they could play.
Electronic and mechanical games have been the exception. As technology advances we see it becoming easier to change at least some of the rules or have pieces that the gamer can put together. From badges, to level editors, to user created content game, developers are allowing the gamer to participate more and more in the creation of the game, and hence the fun.
Re-inventing the ability for the gamer to change the rules of the game has become a priority of game development companies. I look forward to an increasing number of invoative and high quality mods as the line between gamer and modder continues to blur.